Access code detection and dc offset-interference correction

ABSTRACT

A method for detecting an access code in a receiver that does not require an explicit DC-offset interference correction block, comprising:
         a) projecting a received signal R onto a subspace orthogonal to the DC;   b) projecting the access code Ci onto the subspace; and   c) detecting the presence and location of the access code Ci and eliminating any DC offset therein when the frequency offset f 0  in the access code Ci is orthogonal to the subspace, and is therefore nulled out.

The present application for a patent claims priority to provisionalapplication No. 61/101,669 entitled “DC-OFFSET IMMUNE ACCESS CODEDETECTION” filed Sep. 30, 2008, and assigned to the assignee hereof andhereby expressly incorporated by reference herein, and relates generallyto communication systems, and more specifically to access code detectionand DC-Offset interference correction in a wireless communicationsystem.

BACKGROUND Field

Access code detectors detect the presence of an access code in areceived communications signal, and if present, output the location ofthe access code within the received signal. Access code detectors relyon the output of a correlator crossing a fixed threshold in the presenceof the access code. However, a DC offset can cause the output of thecorrelator to malfunction, i.e., cross the threshold in the absence ofan access code and/or cause the output to not cross the threshold in thepresence of an access code.

DC offset is an offsetting of a signal from zero, and is the meanamplitude of the waveform so-if the mean amplitude is zero, there is noDC offset. DC offset is undesirable, i.e. a sound that has DC offsetwill not be at its loudest possible volume when normalized (as theoffset consumes headroom), and this problem can extend to the mix as awhole, because a sound with DC offset and a sound without DC offset willhave DC offset-when mixed.

For example, when implementing a direct conversion receiver, there is anamount of DC offset on the down converted signal due to filter mismatchand due to self-mixing that occurs with the local oscillator (LO)signal, the radio frequency (RF) signal or interfering signals in thereceiver. Filter mismatch due to temperature change over time results instatic DC offset. Self-mixing among the LO, RF and interfering signals,as well as reflection at the antenna, temperature variation and LOleakage also result in dynamic DC offset. Correction for DC offset canbe performed on the variable gain amplifier (VGA) located in thereceiver. Many techniques have been proposed to minimize DC-offset.

Regrettably, one or all of these techniques can only be applied to asystem in which the receiver does not continuously operate, such as in aTDMA communication system.

In a CDMA system, these techniques will not be effective because thereceiver works continuously with no interruption. Also, DC-offsetcorrection using so called “auto-zeroing” techniques during start-up isnot practical in a CDMA system because of dynamic offsets. In a CDMAsystem the only option that shows promise is the implementation of a“servo-loop” like architecture around the variable gain amplifier.

However, in servo-loop architecture, the high pass cut-off frequency isdependent upon the gain characteristics of the variable gain amplifierand the amplifiers in the servo-loop.

Traditionally, access code detectors are preceded by a separateDC-offset correction block. These blocks are often simplistic averagingschemes that rely on known patterns in the transmitted signal.

Therefore, a need exist in the art for an improved access code detectionscheme that does not require an explicit DC-offset correction block thatwill have known patterns in the transmitted signal.

SUMMARY

Embodiments disclosed herein address the above stated needs by utilizingan access code detection scheme that does not require an explicitDC-offset correction block, wherein a received signal is mathematicallyprojected onto a subspace orthogonal to DC, i.e., as an all-one vector.Any received vector affected by a DC-offset is thought of as therequired ideal vector corrupted by a scaled, all-one vector

-   -   a) projecting a received signal R onto a subspace orthogonal to        the DC;    -   b) projecting the access code Ci onto said subspace; and    -   c) detecting the presence and location of said access code Ci        and eliminating any DC offset therein when the frequency offset        f0 in said access code Ci is orthonal to said subspace, and is        therefore nulled out.

As such, projecting the received vector onto a sub-space orthogonal tothe all-one vector removes the effect of the DC-offset.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram illustrating an exemplary vector and its orthogonalsubspace, when considering a n×1 vector V from a n dimensional vectorspace V from the field F, i.e., V ∈ F^(n). The n−1 dimensional subspaceV^(⊥) is the set of vectors perpendicular to the vector V. Therefore,V^(†)V^(⊥)=0.

FIG. 2 is a diagram illustrating an exemplary vector V and itsprojection onto an orthogonal subspace.

FIG. 3 is an exemplary block diagram of an access code detection andDC-offset correction circuitry to perform operations of projecting thereceived signal onto an orthogonal subspace and detect the presence andlocation of an access code.

DETAILED DESCRIPTION

The word “exemplary” is used herein to mean “serving as an example,instance, or illustration.” Any embodiment described herein as“exemplary” is not necessarily to be construed as preferred oradvantageous over other embodiments.

The method and apparatus used in advancement of the art provide accesscode detection and DC-offset correction in a wireless communicationsystem that does not require an explicit DC-offset correction block. Areceived signal is mathematically projected onto a subspace orthogonalto DC, i.e., an all-one vector. Any received vector affected by aDC-offset is considered or thought of as the required ideal vectorcorrupted by a scaled, all-one vector. Consequently, projecting thisreceived vector onto a sub-space orthogonal to the all-one vectorremoves the effect of the DC-offset.

In practice, the received signal is passed through a moving-windowprojector that projects blocks of the received signal onto theorthogonal sub-space. This projected signal is then correlated againstthe projected access-code to detect the presence of access code and thelocation of the access code.

The projector operation is advantageously generic, and does not rely onany known patterns occurring in the access code. Furthermore, theprojection may be accomplished using simple circuitry requiring only amoving window accumulator and a fixed multiplier. The projectionoperation is then followed by a comparator to ensure that the codeproperties of the access codewords are maintained, thereby eliminatingany need for re-calibration of thresholds for the access code detector.

The following exemplary embodiments may replace current access codedetectors by eliminating the need for explicit DC-offset correction andcan be used with any communications technology.

Problem Setting

Let the transmitted access code be denoted by Ci ∈{±1}⁷². The receivedversion of this access code is modeled as R=Ci+Z+f0 (2.1) where Z isadditive noise and f0 is the frequency offset. Given a received signal,the objective is to detect the presence and the location of thetransmitted access code. Access codes have the property that two codeswill differ in at least 14 positions. The access code is viewed as avector. Therefore, Ci is a vector from R⁷². The access code property nowtranslates as E=Ci−Cj:∥E∥² ₂≧4*14 (2.2)

Decoding

A received sequence is detected as the access code if and only if thesquared error between the received sequence and the access code iswithin 4*14, i.e., The access code is detected if and only if

∥R−Ci∥ ² ₂<4*14   (2.3)

The distance between two codewords is inversely proportional to the dotproduct between the codewords which explains why a correlator is aneffective means for detecting an access code, i.e.,

∥E∥ ² ₂ =∥R∥ ² ₂ +∥Ci∥ ² ₂−2R ^(†) Ci=4*72+4*72−2R ^(†) Cr   (2.4)

Detection Scheme

Consider a subspace orthogonal to the all-one vector, as for example inFIG. 1 where there is a diagram illustrating an exemplary vector and itsorthogonal subspace, when considering a n×1 vector V from a ndimensional vector space V from the field F, i.e., V ∈ F^(n). The n−1dimensional subspace V^(⊥) is the set of vectors perpendicular to thevector V. Therefore, V^(†)V^(⊥)=0.

The main idea of this scheme is to project both the received signal Rand the access code Ci onto this subspace.

The main advantage being that the frequency offset f0 is orthogonal tothe subspace and is therefore nulled out, as shown by FIG. 2 where thereis a diagram illustrating an exemplary vector V and its projection ontoan orthogonal subspace.

Let P^(DC) be the orthogonal projector for the subspace orthogonal tothe all one vector, i.e.,

P ^(DC)(f0[11 . . . 1]^(†))=0   (3.1)

Therefore,

P ^(DC) R=P ^(DC) Ci+P ^(DC) Z   (3.2)

This projected received signal is now detected against P^(DC)Ci.

FIG. 3 is a an exemplary block diagram of Access code detection andDC-Offset correction circuitry performing the following operations toproject the received signal onto and orthogonal subspace, and detect thepresence and location of an access code.

Let the transmitted access code be

C _(i) =[c _(i)(1)c _(i)(2)c _(i)(3) . . . c _(i)(K)], where c _(i)(n)∈{−1,+1}.

The received signal is modeled as

R=C _(i) +Z+f ₀, i.e., r(n)=c _(i)(n)+z(n)+f ₀

Z is the additive noise and f₀ is the DC offset.

${r^{''}(n)} = {{{Kr}(n)} - {\sum\limits_{n - K - 1}^{n}{r(k)}}}$${r^{\prime}(n)} = {{Sgn}\left( {{{Kr}(n)} - {\sum\limits_{n - K - 1}^{n}{r(k)}}} \right)}$${r^{''}(n)} = {{{Kr}(n)} - {\sum\limits_{n - K - 1}^{n}{r(k)}}}$${r^{\prime}(n)} = {{Sgn}\left( {{{Kr}(n)} - {\sum\limits_{n - K - 1}^{n}{r(k)}}} \right)}$

r′(n) is the new received signal which is used for regular autocorrelation.

If the transformation of the received signal r(n) to the new receivedsignal r′(n) is given by a mapping r′(n)=f[r(n)]. The followingproperties can be readily observed.

Property 1: The transformed codewords f[C_(i)] have the same distanceproperties as the original codewords C_(i).

Proof:

C_(i) = f[C_(i)]${f\left\lbrack {c_{i}(n)} \right\rbrack} = {{{Sgn}\left( {{{Kc}_{i}(n)} - {\sum\limits_{n - K - 1}^{n}{c_{i}(k)}}} \right)} = {c_{i}(n)}}$

Property 2: The effect of the DC offset is completely eliminated in thenew received signal, regardless of the value of the DC offset.

Proof:

$R^{\prime} = {{f\lbrack R\rbrack} = {{{f\lbrack R\rbrack}_{f_{0} = 0}{r_{i}^{\prime}(n)}} = {{{{Sgn}\left( {{{Kr}_{i}(n)} - {\sum\limits_{n - K - 1}^{n}{r_{i}(k)}}} \right)} + z^{\prime}} = f}}}$$\begin{matrix}{{\left\lbrack {c_{i}(n)} \right\rbrack + {f\left\lbrack {z(n)} \right\rbrack}} = {{f\left\lbrack {c_{i}(n)} \right\rbrack} + {z^{\prime}(n)}}} \\{= {{c_{i}(n)} + {{z^{\prime}(n)}\mspace{14mu} \left( {{from}\mspace{14mu} {Property}\mspace{14mu} 1} \right)}}}\end{matrix}$

Those of skill in the art would understand that information and signalsmay be represented using any of a variety of different technologies andtechniques. For example, data, instructions, commands, information,signals, bits, symbols, and chips that may be referenced throughout theabove description may be represented by voltages, currents,electromagnetic waves, magnetic fields or particles, optical fields orparticles, or any combination thereof.

Those of skill would further appreciate that the various illustrativelogical blocks, modules, circuits, and algorithm steps described inconnection with the embodiments disclosed herein may be implemented aselectronic hardware, computer software, or combinations of both. Toclearly illustrate this interchangeability of hardware and software,various illustrative components, blocks, modules, circuits, and stepshave been described above generally in terms of their functionality.Whether such functionality is implemented as hardware or softwaredepends upon the particular application and design constraints imposedon the overall system. Skilled artisans may implement the describedfunctionality in varying ways for each particular application, but suchimplementation decisions should not be interpreted as causing adeparture from the scope of the present invention.

The various illustrative logical blocks, modules, and circuits describedin connection with the embodiments disclosed herein may be implementedor performed with a general purpose processor, a digital signalprocessor (DSP), an application specific integrated circuit (ASIC), afield programmable gate array (FPGA) or other programmable logic device,discrete gate or transistor logic, discrete hardware components, or anycombination thereof designed to perform the functions described herein.A general purpose processor may be a microprocessor, but in thealternative, the processor may be any conventional processor,controller, microcontroller, or state machine. A processor may also beimplemented as a combination of computing devices, e.g., a combinationof a DSP and a microprocessor, a plurality of microprocessors, one ormore microprocessors in conjunction with a DSP core, or any other suchconfiguration.

The steps of a method or algorithm described in connection with theembodiments disclosed herein may be embodied directly in hardware, in asoftware module executed by a processor, or in a combination of the two.A software module may reside in RAM memory, flash memory, ROM memory,EPROM memory, EEPROM memory, registers, hard disk, a removable disk, aCD-ROM, or any other form of storage medium known in the art. Anexemplary storage medium is coupled to the processor such the processorcan read information from, and write information to, the storage medium.In the alternative, the storage medium may be integral to the processor.The processor and the storage medium may reside in an ASIC. The ASIC mayreside in a user terminal. In the alternative, the processor and thestorage medium may reside as discrete components in a user terminal

The previous description of the disclosed embodiments is provided toenable any person skilled in the art to make or use the presentinvention. Various modifications to these embodiments will be readilyapparent to those skilled in the art, and the generic principles definedherein may be applied to other embodiments without departing from thespirit or scope of the invention. Thus, the present invention is notintended to be limited to the embodiments shown herein but is to beaccorded the widest scope consistent with the principles and novelfeatures disclosed herein.

1. A method for detecting an access code in a receiver that does notrequire an explicit DC-offset interference correction block, comprising:a) projecting a received signal R onto a subspace orthogonal to the DC;b) projecting the access code Ci onto said subspace; and c) detectingthe presence and location of said access code Ci and eliminating any DCoffset therein when the frequency offset f0 in said access code Ci isorthogonal to said subspace, and is therefore nulled out.
 2. The methodof claim 1, wherein: the transmitted access code isC _(i) =[c _(i)(1)c _(i)(2)c _(i)(3) . . . c _(i)(K)], where c_(i)(n)∈{−1,+1}; the received signal is modeled asR=C _(i) +Z+f ₀, i.e., r(n)=c _(i)(n)+z(n)+f ₀; Z is the additive noise;and f₀ is the DC offset.
 3. The method of claim 2, wherein said accesscode has the property that two codes will differ in at least 14positions, and said access code is viewed as a vector from R⁷² and theaccess code property translates as E=Ci−Cj: ∥E∥² ₂≧4*14.
 4. The methodof claim 3, wherein a received sequence is detected as the access codeif and only if the squared error between the received sequence and theaccess code is within 4*14, and the access code is detected if and onlyif∥R−Ci∥ ² ₂<4*14.
 5. The method of claim 4, wherein the distance betweentwo codewords is inversely proportional to the dot product between thecodewords and a correlator is an effective means for detecting an accesscode, as follows:∥E∥ ² ₂ =∥R∥ ² ₂ +∥Ci∥ ² ₂−2R ^(†) Ci=4*72+4*72−2R ^(†) Ci.
 6. Anapparatus for detecting an access code in a receiver that does notrequire an explicit DC-offset interference correction block, comprising:a) means for projecting a received signal R onto a subspace orthogonalto the DC; b) means for projecting the access code Ci onto saidsubspace; and c) means for detecting the presence and location of saidaccess code Ci and eliminating any DC offset therein when the frequencyoffset f0 in said access code Ci is orthogonal to said subspace, and istherefore nulled out.
 7. The apparatus of claim 6, wherein thetransmitted access code isC _(i) =[c _(i)(1)c _(i)(2)c _(i)(3) . . . c _(i)(K)], wherec_(i)(n)∈{−1,+1}; the received signal is modeled asR=C _(i) +Z+f ₀, i.e., r(n)=c _(i)(n)+z(n)+f ₀; Z is the additive noise;and f₀ is the DC offset.
 8. The apparatus of claim 7, wherein saidaccess code has the property that two codes will differ in at least 14positions, and said access code is viewed as a vector from R⁷² and theaccess code property translates as E=Ci−Cj: ∥E∥² ₂≧4*14.
 9. Theapparatus of claim 8, wherein a received sequence is detected as theaccess code if and only if the squared error between the receivedsequence and the access code is within 4*14, and the access code isdetected if and only if∥R−Ci∥ ² ₂<4*14.
 10. The apparatus of claim 9, wherein the distancebetween two codewords is inversely proportional to the dot productbetween the codewords and a correlator is an effective means fordetecting an access code, as follows:∥E∥ ² ₂ =∥R∥ ² ₂ +∥Ci∥ ² ₂−2R ^(†) Ci=4*72+4*72−2R ^(†) Ci.
 11. Acomputer readable media embodying instructions for: a) projecting areceived signal R onto a subspace orthogonal to the DC; b) projectingthe access code Ci onto said subspace; and c) detecting the presence andlocation of said access code Ci and eliminating any DC offset thereinwhen the frequency offset f0 in said access code Ci is orthogonal tosaid subspace, and is therefore nulled out.
 12. The computer readablemedia of claim 11, wherein: the transmitted access code isC _(i) =[c _(i)(1)c _(i)(2)c _(i)(3) . . . c _(i)(K)], where c_(i)(n)∈{−1,+1}; the received signal is modeled asR=C _(i) +Z+f ₀, i.e., r(n)=c _(i)(n)+z(n)+f ₀; Z is the additive noise;and f₀ is the DC offset.
 13. The computer readable media of claim 12,wherein said access code has the property that two codes will differ inat least 14 positions, and said access code is viewed as a vector fromR⁷² and the access code property translates as E=Ci−Cj: ∥E∥² ₂≧4*14. 14.The computer readable media of claim 13, wherein a received sequence isdetected as the access code if and only if the squared error between thereceived sequence and the access code is within 4*14, and the accesscode is detected if and only if∥R−Ci∥ ² ₂<4*14.
 15. The computer readable media of claim 14, whereinthe distance between two codewords is inversely proportional to the dotproduct between the codewords and a correlator is an effective means fordetecting an access code, as follows:∥E∥ ² ₂ =∥R∥ ² ₂ +∥Ci∥ ² ₂−2R ^(†) Ci=4*72+4*72−2R ^(†) Ci.
 16. A remotestation apparatus for detecting an access code in a receiver that doesnot require an explicit DC-offset interference correction block,comprising: a) means for projecting a received signal R onto a subspaceorthogonal to the DC; b) means for projecting the access code Ci ontosaid subspace; and c) means for detecting the presence and location ofsaid access code Ci and eliminating any DC offset therein when thefrequency offset f0 in said access code Ci is orthogonal to saidsubspace, and is therefore nulled out.
 17. The remote station apparatusof claim 16, wherein the transmitted access code isC _(i) =[c _(i)(1)c _(i)(2)c _(i)(3) . . . c _(i)(K)], where c_(i)(n)∈{−1,+1}; the received signal is modeled asR=C _(i) +Z+f ₀, i.e., r(n)=c _(i)(n)+z(n)+f ₀; Z is the additive noise;and f₀ is the DC offset.
 18. The remote station apparatus of claim 17,wherein said access code has the property that two codes will differ inat least 14 positions, and said access code is viewed as a vector fromR⁷² and the access code property translates as E=Ci−Cj: ∥E∥² ₂≧4*14. 19.The remote station apparatus of claim 18, wherein a received sequence isdetected as the access code if and only if the squared error between thereceived sequence and the access code is within 4*14, and the accesscode is detected if and only if∥R−Ci∥ ² ₂<4*14.
 20. The remote station apparatus of claim 19, whereinthe distance between two code words is inversely proportional to the dotproduct between the code words and a correlator is an effective meansfor detecting an access code, as follows:∥E∥ ² ₂ =∥R∥ ² ₂ +∥Ci∥ ² ₂−2R ^(†) Ci=4*72+4*72−2R ^(†) Ci.